The term security mark is to denote a bit sequence, which is for example used for providing an authentication key or decrypting key on a data carrier. Authentication and encryption are commonly used on data carriers in order to prevent unauthorized copying of information, which is stored on the data carrier and protected by copyright law. The authentication key is used for establishing or confirming that the data carrier contains a legally sold copy and has not been copied in breach of copyright protection. The readout of data from the data carrier is prevented if the authentication key is inexistent or incorrect. Alternatively, the data may be encrypted, such that a decrypting key is necessary for obtaining the stored information from the read out bit sequences.
The security mark may only protect unauthorized copying and reproduction of data stored on a data carrier if the security mark may not be copied using standard disk drives. Conventional disk drives such as Compact Disks (CD), Digital Versatile Disks (DVD) or BluRay Disks (BD) use a focused laser beam for storing data on the disk. Thereby, a series of microscopic indentations (“pits”, with the gaps between them referred to as “lands”) are formed in the recording area of the disk. The laser beam is directed onto a reflective surface of the disk to read the pattern of pits and lands. The pattern of changing intensity of the reflected beam is converted into binary data.
The international patent application WO 2005/048256 discloses using a hologram, from which a key or authentication mark can be derived. A hologram is an advanced form of photographic recording that allows an image to be recorded in three dimensions. The technique of holography can also be used to optically store, retrieve, and process information. To produce a recording of the phase of the light wave at each point in an image, holography uses a reference beam which is combined with the light from the scene or object (the object beam). Optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction pattern on the film, which is called the hologram or the interference pattern. Therefore two coherent light beams, an object beam and a reference beam, are necessary for recording a hologram. A conventional disk drive does not have the means for recording a hologram. The technical equipment and know how for copying a holographic recording is complex and expensive. Furthermore, the analysis and reproduction of the diffraction pattern on the film is a formidable task. Therefore, unauthorized copying and reproduction of the security mark is impeded.
However, the security mark stored as a holographic recording may not be readout using a standard disk drive. According to the international patent application WO 2005/048256 a unique spatial modulation filter must be applied at the level of the analysis of the security mark. Therefore, the holographic security mark according to the state of the art necessitates a difficult and expensive modification of the disk drive in order to be readable. The data carriers having these holographic security marks are not compatible with existing disk drives.